If you go to the app store and look at the reviews for TripAdvisor, you'll see not reviews of the app, but reviews of hotels and restaurants. Tens of thousands of them.

Back in 2010, ReadWrite wrote a small piece about Facebook's login, and this somehow made its way onto the first page of Google search results for 'Facebook login'. And it had hundreds of comments from people saying 'I don't understand - how do I login in?'. They'd clicked on the link, arrived on ReadWrite, and thought they were on Facebook.

On one level, these kinds of stories remind me of the old joke that you can drive a car perfectly well if you think it's powered by little horses somewhere inside. Once control and operation have been abstracted enough that you don't have to crank a starter handle or feed a punch card into a slot, you don't need to know exactly how every layer of the stack works, at least to some degree - you have to understand wheels, but not internal combustion. But sometimes you get artefacts - sometimes the gap in understanding shows through.

There are deeper things to think about, though, around how we build mental models to cope with these issues. A few years ago, one of the big UK retailers told me an anecdote from some market research they’d done into cameras. Their customers had said they wanted a solution for storing all the camera cards they had. This puzzled the researchers, so they dug a little further, and found out that a lot of their customers had dozens and dozens of memory cards.

Why would you have dozens of memory cards for your camera?

Well, you went to the shop and bought a new camera, and you got a digital one whether you wanted one or not. So you asked the shop assistant what you used for film with a digital camera, and they sold you a memory card. When it was full, some people took it somewhere that could print the photos for you - a print kiosk, probably - but not until it was full, because once you’ve printed them you can’t put more images on the card. It's as though you’ve developed a roll of film. Other people just took the memory card out of the camera at the end of a trip, and when they wanted to show people the photos they’d taken they retrieved the card and put it back into the camera.

I recognise this behaviour because it’s what my father-in-law does - and when he wants to print something from his computer, he takes a photo of the screen, takes out the camera's memory card, slots it into the printer and prints out the photo (he also made quite a lot of money day-trading Imagination Tech, over the phone).

As we go from 1.5bn PCs (of which only half are consumer) to 3bn iOS and Android devices today and 4-5bn in the future, this will become ever more important. We’re making devices that are more personal and important with more at stake, and we’re also giving them more and more to people who’ve never used a computer before. Indeed, a lot of the ‘next billion’ have never owned an electronic device before, and some have never owned an electrical device at all, except perhaps a radio. How much abstraction is right in this world, and is it helpful or unhelpful to have prior mental models (‘memory cards are like film’) to apply?

We're all prone to apply old mental models to new things when they look like the old things. Cameras are cameras, and the card is film, so you treat the card somewhat like you treat film. The challenge for a new thing is that you can fall into one of two traps - either you try to map it to the old mental model, or you decide that, since it has no existing mental model, it's useless. So, the automobile is compared to the carriage, Uber is compared to taxis, digital cameras to film cameras, and smart watches to Rolexes. But sometimes there is no model. So my father-in-law experiences a different world to us - he stands in the same place and sees something different. But all of us have that same disconnect whenever we try to understand something new.

I work at Andreessen Horowitz ('a16z'), a venture capital firm in Silicon Valley that invests in software companies. I try to work out what's going on and what will happen next.

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